Various document processing machines such as printing machines need to align a sheet with its corresponding image prior to transferring the image to the sheet. Proper alignment is needed in order to accurately transfer the image to the correct location on the sheet. Many elements of a machine such as a platen, belt, or image transfer element are static or have fixed motions and therefore not amenable to realignment. The sheet, however, is dynamic. Therefore, many printing machines contain a registration device that realigns the sheet in order to facilitate accurate placement of the image transfer onto the sheet.
As a sheet moves through a printer, there is a desired path over which the sheet travels in order to remain in proper alignment for printing. Typically, the trajectory of the traveling sheet tends to diverge from the desired path. This divergence can be caused by many factors such as a sheet being fed incorrectly into a printer, or a sheet being at an angle on the feed tray, etc. The divergence is corrected by way of the registration device.
Several types of registration devices exist which utilize different means of realigning a sheet. For example, certain registration devices are mechanistic using a combination of direct current and stepper motors to drive the sheet. Others use a split nip shaft where each drive roller is driven by a separate motor. Another device uses nips as well as a cross-process motor, which alters movement along the y-axis of a sheet.
To correct this divergence, most registration devices contact the sheet at two points or “nips” and have 3 or more degrees of freedom. The degrees of freedom of the sheet in the plane of sheet travel include: process direction, cross-process direction, and angular velocity. In order to have accurate sheet registration, all three degrees of freedom must be identified and accurately adjusted. The velocity vectors at the two contact points of registration devices with four or more degrees of freedom induce an additional factor, buckling or stretching of the sheet between the two contact points. If the velocity vectors at the two contact points are not controlled correctly, the sheet may buckle or stretch in a cross-process direction between the contact points of the registration device. Buckling or stretching can lead to sheet jams and/or sheet damage. Hence, for registration devices with 4 or more degrees of freedom, it is important that the buckling/stretching between the two contact points is properly controlled.
Typically, a registration controller locates the sheet with respect to the position of the image. For example, the sheet may be sensed and signals sent to a registration device to cause the sheet to position accurately in relation to the image. A single controller typically receives sensor information and alters the velocity of nips accordingly.
Registration devices come in varying configurations and may each require their own specific control algorithm to properly operate. The printing machine having a registration device includes controls for tracking the sheet, calculating the sheet's present trajectory, and providing signals to the registration device to correct deviations from the theoretical path. Such information is operated on by an algorithm specific for the registration device. The controls for the registration device and for tracking the sheet and determining its trajectory are typically intertwined with the algorithm controlling the registration device. If the registration device is replaced with a different type of registration device, perhaps for maintenance or upgrading purposes, the entire control mechanism and software need to be modified to accommodate the new device. Depending on the machine, such modification may not be possible and the choice of registration devices would be limited.